1. Technical Field
The present invention relates to a device and the like for controlling a tape drive which drives a tape storage medium. More specifically, the present invention relates to a device and the like which have the function of issuing notification of the approach of the end of a tape when the end is being approached.
2. Description of the Related Art
A tape drive which performs writing to and reading from a tape generally has the function of issuing notification of the fact that the end is being approached. The fact that the end of tape is being approached is notified, for example, as an error for a write command from a host computer (hereinafter referred to as a “host”). It should be noted that in this specification, the end of the tape is referred to as a PEOT (physical end of tape) and a position on the tape at which notification of the approach of the PEOT is issued is referred to as an LEOT (logical end of tape).
When such notification has been issued, the tape drive must record on the tape all data transmitted from the host by that time.
Generally, the tape drive has a large buffer memory in order to improve performance. Data transmitted from the host is temporarily stored in this buffer memory and then recorded on the tape. Accordingly, at the point in time when notification of the approach of the PEOT is issued, a space in which all data stored in the buffer memory can be recorded is left on the tape.
Heretofore, a capacity equivalent to the buffer size has been consistently employed as such a space. That is, the LEOT has been determined such that a space in which data of a volume equivalent to the buffer size is typically left before the PEOT.
As described above, in conventional technology, the LEOT is typically determined such that a space of uniform size is provided between the LEOT and PEOT if a tape. For example, a position approximately 15 m before the PEOT may be determined to be and indicated as the LEOT.
However, if the LEOT is uniformly determined as described above, there arises the problem that a recording area on the tape may be wasted which increases as the buffer memory size increases. In particular, the storage capacity of the tape is wasted where there is little data to be written from the buffer memory as will be described with reference to FIG. 11A.
In FIG. 11A, it is assumed that the buffer memory has a capacity of 256 data sets and that the tape length per data set is 70 mm. In this case, a tape length for the case where all data sets in the buffer memory are recorded is 70 mm □ 256=approximately 18 m. Accordingly, the LEOT is set at a position approximately 28 m before the PEOT so that a margin of 10 m is left.
In FIG. 11A however, it is further assumed that only one data set is actually stored in the buffer memory. In this case, data is recorded only in a tape space indicated by the black strip portion in the drawing. Thus, a tape space indicated by the white strip portion is wasted.
Further, there are cases which are prevalent in conventional tape storage systems where data is recorded while areas having damage or the like on a tape are skipped, depending on standards for the format of the tape. For example, such data recording is permitted in systems implementing the Linear Tape-Open (LTO) standard, jointly-developed by International Business Machines (IBM) Corporation of Armonk, N.Y., Hewlett-Packard Corporation of Palo Alto, Calif., and Quantum Corporation of San Jose, Calif. (formerly Seagate Technology). In LTO, a skippable tape length is 4 m at the maximum. Accordingly, if the maximum skip is performed, the tape length per data set becomes 4 m. On the other hand, if a skip is not performed, the tape length per data set is only 70 mm.
In such circumstances, it is supposed that the occurrence of the need for skips in all data sets is very unlikely. Consequently, a tape length obtained by adding some space margin for the case where skips are assumed not to be performed is employed as a space to be left when the LEOT is determined. However, the frequency at which skips occur can increase more than is anticipated by the described space margin. Accordingly, there exists a risk inherent in using such a method of determining the LEOT that not all data sets in the buffer memory may be written to tape as will be described with reference to FIG. 11B.
In FIG. 11B, the LEOT is also set at a position approximately 28 m before the PEOT based on a concept similar to that of FIG. 11A. However, if skips frequently occur as illustrated in the drawing, some data cannot be written as indicated by the black strip left of the PEOT.